1. Technical Field
The present invention relates to temperature sensing to be applied in a semiconductor memory device, and more particularly, to a method for outputting internal temperature data in a volatile semiconductor memory device such as Dynamic Random Access Memory (DRAM) and a circuit for outputting the internal temperature data thereby.
2. Discussion of Related Art
Generally, for high-efficiency electronic systems such as personal computers or electronic communication machinery, volatile semiconductor memory devices such as DRAM onboard as memory have become faster and more highly integrated. In case of semiconductor memory devices onboard in battery-operated systems such as mobile phones or laptop computers, the low power consumption characteristics are critically required. Therefore, semiconductor manufacturers have continuously worked to reduce the operating current and standby current in order to provide a mobile oriented low power solution.
The data retention characteristics of the memory cell in the DRAM comprising one transistor and one storage capacitor are very sensitive to temperature. Thus, if the memory cell can be controlled suitably according to its temperature characteristics, this can be useful in saving power. An approach to realize such power saving by installing a temperature sensor in the semiconductor memory device such as the DRAM and differentiating a refresh cycle according to the temperature of a chip is well known in this art.
FIG. 1 illustrates a circuit 100 of a temperature sensor to be installed in a semiconductor memory device. The temperature sensor is a semiconductor temperature sensor of a band gap reference type basically comprising a current mirror-type differential amplifier and a diode, which is well known in this art. Currents flowing in branches C and A of the temperature sensor have temperature-current characteristics as shown in FIG. 2. In FIG. 2, the horizontal axis indicates temperature and the vertical axis indicates current. The characteristic graphs of the branches C and A intersect at trip point T1.
However, the temperature sensor as described above is very sensitive to a noise environment, and thus the deviation of the temperature data as output according to the operation modes of the semiconductor memory device may be great. Consequently, there are problems in that accuracy of the obtained temperature data is lowered and reliability of the temperature data is also lowered accordingly.
In practice, an attempt has been made to transfer the temperature of the DRAM chip into a chip-set such as a CPU or memory controller so that the chip-set controls various operations of the DRAM, for example, a refreshing operation. In such a case, the temperature sensor is continuously or periodically activated to perform a temperature sensing operation. When an external command is applied like a waveform (Command) in FIG. 3 while the temperature sensor is periodically operating, internal temperature data is read-out in the DRAM until a transition of a waveform (TS-RD) occurs. FIG. 3 illustrates temperature data output timing according to the conventional art.
When the internal temperature data of the DRAM chip is obtained by the manner shown in FIG. 3, the temperature data obtained according to various operation modes of the DRAM may have differences. That is, if the external command is applied when the DRAM performs an operation of reading the data, it is difficult for the temperature sensor to sense the present temperature data in a sufficiently stable state, and it may output deviated temperature data due to the noise environment. Consequently, the reliability of the internal temperature data as obtained is lowered, and power consumption is increased since the temperature sensor is in the continuously or periodically operating state. Moreover, since an output access time of the temperature data is indicated as access section TA of FIG. 3, there is a problem in that the time for the chip-set to obtain the temperature data becomes somewhat long.
In order to obtain accurate temperature data, it is preferable to ensure that the environment is without any noise, during which the temperature sensor can perform the temperature sensing operation in a sufficiently stable state, for more than the time for responding to the sensing of the temperature sensor. Thus, measures to make it possible to obtain more reliable temperature data within a shorter time, without damaging/interrupting the performance of the semiconductor memory device, are required in the battery-operated systems.